This invention relates to an optical semiconductor device with resonant cavity tunable in wavelength.
This device may equally well be made in the form of a coherent light source, for example a vertical emission laser also called VCSEL in the rest of this document, or an edge emission laser, or in the form of an incoherent light source, for example a light emitting diode also called LED throughout the rest of this document and with vertical emission or edge emission.
The device according to the invention may also be made in the form of an optical modulator operating in reflection or in transmission.
The invention is used in a variety of fields, for example such as optical telecommunications, all optical networks, information display and spectroscopy.
Note that in a vertical emission diode, the mirrors that delimit the resonant cavity of the diode are formed perpendicular to the growth axis of the layers surrounding the active area and the spacer located in the cavity. This spacer is itself between the active area and the cavity mirrors.
The length of this cavity may be of the order of 1 xcexcm and it is then called a xe2x80x9cmicro-cavityxe2x80x9d. A single optical mode is usually selected in this type of micro-cavity due to the thinness of the micro-cavity, although there may be several optical modes in a thicker resonant cavity.
Vertical emission diodes of the VCSEL or LED type offer enormous potential for the optical telecommunications market due to:
their ease of manufacturing, so that matrices can be made with a large number of light emitters,
the quality of the electromagnetic mode of their light emission, and
their ease of integration into optoelectronic circuits.
Vertical emission laser diodes that are tunable by temperature variation are already known in document [1] which, like the other documents mentioned below, is included in the references at the end of this description. This type of diode has a very long response time, of the order of 1 second, and a narrow tunability range.
Document [2] also describes a vertical emission diode tunable using a deformable membrane that is electrically controlled and forms the output mirror from the resonant cavity of the diode, which may have a tunability of several tens of nanometers. However, it is difficult to make and consequently is fairly expensive. Furthermore, the use of a deformable membrane gives a fairly long response time, of the order of 1 xcexcs.
Document [3] describes laser diodes tunable by temperature variation, for edge emission diodes. This type of diode has a very long response time, of the order of 1 second, and a fairly limited tunability range.
Document [4] also describes edge emission laser diodes that are tunable by means of an external diffraction grating. The response time of this type of diode is of the order of 1 millisecond and they are also large and very expensive.
Furthermore, document [5], to which we will refer, describes tunable edge emission laser diodes in which the resonant cavity comprises an active region, a region with a variable refraction index (phase shifter) and a region forming a distributed Bragg reflector (DBR), these regions being included monolithically on a substrate.
When a current passes through the active region, a stimulated emission effect is obtained with a gain spectrum over a fairly wide range. A large number of cavity nodes is available within this range. Nevertheless, since the DBR has a single maximum at wavelength xcexf, only one cavity mode can actually lead to laser radiation. In this configuration, tunability is obtained by allowing a current to pass through the DBR, which changes the refraction index in this DBR by a plasma effect and therefore offsets xcexf, in other words the emission wavelength.
Despite this offset, cavity modes remain practically unchanged and therefore the emission passes discontinuously from one mode to the other. The region with a variable refraction index is used to prevent this effect, and is controlled by an electrical current independently of the DBR. This change in the refraction index is sufficient to offset cavity modes that can then follow the variation of xcexf. This can give a continuous variation of the emission wavelength.
The response time of this type of diode is directly related to the lives of charge carriers in the region with variable refraction index, and in the region of the DBR, and is equal to about 1 nanosecond.
However, these diodes are difficult to make and therefore expensive and also require complex electronic means to control the laser radiation wavelength since three electrical currents are necessary for this purpose (one current for each of the regions mentioned above).
The purpose of this invention is to correct the disadvantages mentioned above by proposing an optical semiconductor device with resonant cavity tunable in wavelength which is easier to make and therefore less expensive than the devices described in documents [2] and [5], but is still fast, this device being likely to have a response time, in other words a wavelength switching time, of the order of 1 nanosecond or even less.
More precisely, the purpose of this invention is an optical semiconductor device with a resonant cavity tunable in wavelength, this device comprising a resonant cavity and two mirrors that delimit this cavity, this device being characterized in that it also comprises:
at least one super-lattice that is placed in the cavity and is formed starting from piezoelectric semiconducting layers, and
first means of injecting charge carriers into the super-lattice,
so that the optical properties of this super-lattice can be modified when the charge carriers are injected into it thus creating an offset in the wavelength of the cavity resonance modes.
In this document, the xe2x80x9cpiezoelectricxe2x80x9d effect should be understood in the broad sense and thus also includes phenomena in which an electric field appears in a strained semiconducting layer, and spontaneous polarization phenomena in semiconducting layers of the wurzite type in which an electric field can occur without any strain being applied to these layers. Further information on this subject can be found in document [6].
The device may also comprise an active area placed in the cavity and designed to emit coherent radiation during the injection of charge carriers into this active area.
This radiation emission may equally well be coherent emission (laser) or incoherent emission.
According to a first particular embodiment of the device according to the invention, the first charge carrier injection means are also designed to inject these charge carriers into the active area so that this active area emits radiation.
According to a second particular embodiment, the device also comprises second charge carrier injection means designed to inject these charge carriers into the active area so that this active area emits radiation.
These charge carriers are then independent of carriers that are injected into the super-lattice that is formed starting from piezoelectric semiconducting layers and subsequently denoted SR-P.
In a first example, the device according to the invention may emit laser radiation, the mirrors being perpendicular to the growth axis of the semiconducting layers included in the device, in order to obtain vertical emission of the laser radiation.
In a second example, the device according to the invention may emit incoherent radiation, the mirrors being perpendicular to the growth axis of the semiconducting layers included in the device in order to obtain vertical emission of incoherent radiation.
In a third example, the device according to the invention can emit laser radiation, the mirrors being parallel to the growth axis of the semiconducting layers included in the device, in order to obtain edge emission of the laser radiation.
In a fourth example, the device according to the invention may emit incoherent radiation, the mirrors being parallel to the growth axis of the semiconducting layers included in this device, in order to obtain edge emission of the incoherent radiation.
The first, third and fourth examples above are applicable in particular to optical telecommunications, whereas the second example is applicable in particular to information display and spectroscopy (detection of gases or pollutants).
The first means of injecting charge carriers may be electrical.
As a variant, these first injection means may be optical.
Piezoelectric semiconducting layers may have zinc blende type crystalline structures.
As a variant, these piezoelectric semiconducting layers may have wurtzite type crystalline structures.
In a particular embodiment of the device according to the invention, this device does not include an active layer in its resonant cavity and forms an optical modulator that operates in reflection or in transmission and is capable of modulating incident light using the variation of cavity modes. The conducting or non-conducting state of the modulator depends on the position of these modes. This modulator is used particularly for optical telecommunications and all-optical network applications.
The modulation rate of a device according to the invention may be greater than 1 GHz for a tunability of the order of a few nanometers.
The invention also relates to a light intensity modulation device comprising an optical device according to the invention that comprises an active area placed in the resonant cavity of this optical device, this optical device being optically coupled to another resonant cavity without an active area but in which a super-lattice is placed formed from piezoelectric semiconducting layers, this other cavity being capable of resonating at one or more tunable wavelengths.
The invention also relates to a light intensity modulation device comprising an optical device according to the invention that includes an active area placed in the resonant cavity of this optical device, which is optically coupled to another resonant cavity without an active area or super-lattice formed from piezoelectric semiconducting layers, this other cavity being capable of resonating at one or a plurality of fixed wavelengths.